Genetics: DNA, Chromosomes, Cell Cycle, and Chromosomal Variation

Comprehensive practice flashcards covering DNA structure, chromatin organization, cell cycle, mitosis, meiosis, gametogenesis, sex determination, aneuploidy, polyploidy, and chromosomal rearrangements.

What three chemical components constitute a single repeating nucleotide unit in DNA and RNA?

A pentose sugar, a phosphate group, and a nitrogenous base.

What chemical difference distinguishes ribose from deoxyribose at the $2'$ carbon atom?

Ribose has a hydroxyl group ($-\text{OH}$) at the $2'$ carbon, whereas deoxyribose has a hydrogen atom ($-\text{H}$) at the $2'$ carbon.

To which carbon atom of the pentose sugar is the phosphate group always attached in a nucleotide?

The $5'$ carbon atom.

What charge is carried by the phosphate group in a polynucleotide chain?

A negative charge.

Which nitrogenous bases are classified as purines?

Adenine (A) and Guanine (G).

Which nitrogenous bases are classified as pyrimidines?

Cytosine (C), Thymine (T), and Uracil (U).

Which pyrimidine nitrogenous base is present exclusively in DNA, and which is present exclusively in RNA?

Thymine (T) is present in DNA, while Uracil (U) is present in RNA.

Identify the four specific deoxyribonucleotides shown in the structural diagram.

Deoxyadenosine $5'$\text{-monophosphate} (dAMP), Deoxyguanosine $5'$\text{-monophosphate} (dGMP), Deoxythymidine $5'$\text{-monophosphate} (dTMP), and Deoxycytidine $5'$\text{-monophosphate} (dCMP).

Why does double-stranded DNA maintain a constant width along its helix?

Because a two-ring purine always pairs with a single-ring pyrimidine.

What are Chargaff's rules regarding base composition in double-stranded DNA?

The ratio of Adenine to Thymine equals $1$ ($A/T = 1$), the ratio of Guanine to Cytosine equals $1$ ($G/C = 1$), and the amount of purines equals pyrimidines ($A + G = T + C$).

What historical X-ray diffraction photograph provided key structural evidence for the DNA double helix?

Photo 51 (taken by Rosalind Franklin and Raymond Gosling).

What chemical bonds connect adjacent nucleotides along a single strand of DNA?

Phosphodiester bonds between the $5'\text{-PO}_4$ group of one nucleotide and the $3'\text{-OH}$ group of the next.

How many hydrogen bonds form between Adenine and Thymine versus Guanine and Cytosine?

$A\text{-}T$ base pairs are joined by $2$ hydrogen bonds, whereas $G\text{-}C$ base pairs are joined by $3$ hydrogen bonds.

Why does DNA with higher $G\text{-}C$ content require a higher melting temperature ($T_m$) to denature?

Because $G\text{-}C$ base pairs have three hydrogen bonds compared to two in $A\text{-}T$ pairs, requiring more energy to disrupt.

What physical length units correspond to $1{,}000$ base pairs and $1{,}000{,}000$ base pairs, respectively?

$1$ kilobase pair (Kb or Kbp) equals $1{,}000$ bp, and $1$ megabase pair (Mb or Mbp) equals $1{,}000$ Kb or $1{,}000{,}000$ bp.

What is the structural difference between a hairpin (stem-loop) and a stem secondary structure in single-stranded nucleic acids?

A hairpin consists of a paired stem with a loop of unpaired bases at its end, whereas a stem consists only of paired bases without a loop.

In the secondary structure of double-stranded DNA, where are the sugar-phosphate backbone and base pairs positioned?

The sugar-phosphate backbone is located on the outside, and the complementary base pairs are located on the inside.

What enzymes alter DNA tertiary coiling by introducing or removing rotations through breaking and rejoining strands?

Topoisomerases.

What tertiary structural state occurs when circular bacterial DNA undergoes additional winding or twisting?

Supercoiling.

What is chromatin?

The complex of double-stranded DNA and associated proteins that composes eukaryotic chromosomes.

Approximately what fraction of the total protein mass in chromatin consists of histone proteins?

About half of the total protein mass (chromatin itself is roughly half DNA and half protein by weight).

What charge do histone proteins carry, and what is its functional significance?

Histones carry a positive charge, enabling them to bind electrostatically to the negatively charged sugar-phosphate backbone of DNA.

Which specific histone protein binds to the outside of the nucleosome core particle to form a chromatosome?

Histone H1.

What are the diameters of the chromatin fibers at progressive levels of compaction shown in the diagram?

$2\,\text{nm}$ DNA double helix $\rightarrow$ $11\,\text{nm}$ nucleosomes $\rightarrow$ $30\,\text{nm}$ fiber $\rightarrow$ $300\,\text{nm}$ loops $\rightarrow$ $250\,\text{nm}$/$700\,\text{nm}$ chromatid $\rightarrow$ $1400\,\text{nm}$ mitotic chromosome.

What is satellite DNA?

Highly repetitive heterochromatin DNA consisting of tandem repeats $< 10\,\text{bp}$ long clustered in millions of copies at centromeres and telomeres.

What specific hexanucleotide repeat sequence constitutes human telomeres?

$5'\text{-TTAGGG-}3'$, repeated $100$ to $1{,}000$ times per telomere.

What structure is created when the single-stranded G-rich $3'$ overhang at a telomere loops back and pairs with the double-stranded DNA?

A t-loop.

Identify the four categories of eukaryotic chromosome morphology defined by centromere location.

Metacentric (centromere in middle), Submetacentric (off-center), Acrocentric (near end with small p arms), and Telocentric (at telomere, no visible small arms).

What are the main functions of telomeres on eukaryotic linear chromosomes?

To maintain chromosomal end integrity, protect ends from degradation, and prevent end-to-end chromosome fusion.

What is the difference between dioecious species and monoecious species?

Dioecious species have separate male and female individuals, whereas monoecious species have both male and female reproductive structures in the same individual.

What is the difference between autosomes and sex chromosomes?

Autosomes are chromosomes not involved in sex determination, while sex chromosomes directly determine sexual phenotype.

What is a karyotype?

A microscopic display showing an individual's complete set of metaphase chromosomes ordered by size and homologous pairing.

Define chromosomal sex versus phenotypic sex.

Chromosomal sex is established at fertilization by sex chromosome composition; phenotypic sex is the internal/external morphology arising from gene expression.

What are the diploid ($2n$) chromosome numbers for Homo sapiens, Pan troglodytes, and Drosophila melanogaster?

Homo sapiens: $2n = 46$; Pan troglodytes: $2n = 48$; Drosophila melanogaster: $2n = 8$.

Which organism possesses the highest known chromosome number mentioned in the notes?

The fern Ophioglossum, with $n = 630$ ($2n = 1260$).

In the XX-XY sex-determination system, which sex is homogametic and which is heterogametic?

Females are homogametic (XX) and males are heterogametic (XY).

What term describes a male possessing only one copy of an X-linked gene?

Hemizygous.

In the ZZ-ZW sex-determination system, which sex is heterogametic and which is homogametic?

Females are heterogametic (ZW) and males are homogametic (ZZ).

Which organism groups utilize the ZZ-ZW sex-determination system?

Birds, butterflies, some reptiles, and some amphibians.

What are pseudoautosomal regions (PAR1 and PAR2)?

Small homologous regions at the tips of mammalian X and Y chromosomes that allow pairing and recombination during male meiosis.

What gene located on the human Y chromosome triggers male development?

The SRY gene (Sex-determining region Y).

What molecular role does the product of the SRY gene perform?

It acts as a transcription factor required for male-specific gene expression, directing the indifferent gonad to form testes.

What phenotypic sex develops in individuals with XXY and XO sex chromosome compositions?

XXY individuals develop as phenotypic males, and XO individuals develop as phenotypic females.

What is the chromosomal constitution and incidence of Turner syndrome?

XO monosomy, occurring in approximately $1$ in $3{,}000$ live female births.

What is the chromosomal constitution and incidence of Klinefelter syndrome?

XXY (or XXXY, XXXXY, XXYY), occurring in approximately $1$ in $1{,}000$ live male births.

What genetic mechanism can give rise to XY females or XX males?

Translocation of the SRY gene onto an X chromosome (causing XX males) or mutation/deletion of the SRY gene (causing XY females).

What is a gynandromorph?

An individual possessing both male and female genetic/tissue characteristics, often created by mitotic nondisjunction during early development.

What offspring sex ratio is produced from fertilization between an XY male and an XX female?

A $1:1$ sex ratio ($50\%$ XX female and $50\%$ XY male).

What two main divisions make up the eukaryotic cell cycle?

Interphase ($G_1$, S, $G_2$) and M phase (Mitosis and Cytokinesis).

What key event takes place during the S phase of interphase?

DNA replication (chromosome duplication).

How does the physical state of chromatin compare between interphase and M phase?

Interphase chromatin is relaxed and uncoiled (invisible under light microscope), whereas M phase chromosomes are highly condensed and visible.

List the five stages of mitosis in correct sequential order.

Prophase, Prometaphase, Metaphase, Anaphase, and Telophase.

What major cellular changes happen during mitotic prophase?

Chromosomes condense to become visible, and the mitotic spindle begins to form from centrosomes.

What key events occur during mitotic prometaphase?

The nuclear envelope disintegrates, and spindle microtubules attach to chromosome kinetochores.

Where do chromosomes align during metaphase of mitosis?

Along the metaphase plate (equatorial plane) between spindle poles.

What separates during anaphase of mitosis?

Sister chromatids separate at their centromeres, becoming individual daughter chromosomes that move to opposite poles.

What events mark the completion of mitosis during telophase?

Daughter chromosomes reach spindle poles, nuclear envelopes re-form, and condensed chromatin relaxes.

Define cytokinesis.

The division of cytoplasm resulting in two distinct daughter cells.

How do daughter cells produced by mitosis compare to those produced by meiosis in number and ploidy?

Mitosis produces $2$ genetically identical diploid ($2n$) cells; Meiosis produces $4$ genetically non-identical haploid ($n$) cells.

Why is mitosis termed equational division and Meiosis I termed reductional division?

Mitosis preserves chromosome number ($2n \rightarrow 2n$), whereas Meiosis I reduces chromosome number by half ($2n \rightarrow n$) by separating homologous pairs.

When specifically during meiosis does crossing over take place?

In late Prophase I of Meiosis I.

Between which chromatids within a homologous pair does crossing over occur?

Between non-sister chromatids of homologous chromosomes.

What are chiasmata?

X-shaped structures representing physical crossover sites between non-sister chromatids.

What gamete genotypes are produced from meiosis of a dihybrid ($AB/ab$) when crossing over occurs between linked genes?

Two nonrecombinant genotypes ($AB$, $ab$) and two recombinant genotypes ($aB$, $Ab$).

What structures align at the metaphase plate in Metaphase I of Meiosis I versus Metaphase of Mitosis?

Homologous chromosome pairs align in Metaphase I of Meiosis I, whereas individual chromosomes align in Metaphase of Mitosis.

What structures separate during Anaphase I of Meiosis I versus Anaphase II of Meiosis II?

Homologous pairs separate in Anaphase I, whereas sister chromatids separate in Anaphase II.

What is interkinesis?

The period between Meiosis I and Meiosis II during which nuclear membranes may re-form and chromosomes relax, but no DNA replication occurs.

What two mechanisms in meiosis generate genetic variation in gametes?

Crossing over in Prophase I and random alignment/independent assortment of maternal and paternal chromosomes in Metaphase I.

Compare somatic cells and germ cells regarding their participation in meiosis.

Somatic cells never undergo meiosis (only mitosis), whereas germ cells located in gonads are the only cells that undergo meiosis.

Where are germ cells exclusively located in animals?

In the gonads (testes in males, ovaries in females).

What diploid cells continuously divide by mitosis in the adult male testes to maintain sperm production?

Spermatogonia.

Outline the cell lineage of human spermatogenesis from primary spermatocyte to mature sperm.

Primary spermatocyte ($2n$) $\rightarrow$ Meiosis I $\rightarrow$ $2$ secondary spermatocytes ($1n$) $\rightarrow$ Meiosis II $\rightarrow$ $4$ spermatids ($1n$) $\rightarrow$ maturation $\rightarrow$ $4$ sperm.

During what developmental stage does mitotic proliferation of oogonia occur in human females?

Exclusively during embryonic/fetal development.

At what stage of meiosis do primary oocytes arrest in human females prior to ovulation?

Prophase I of Meiosis I.

What two cellular products are generated when a primary oocyte completes Meiosis I upon ovulation?

One large secondary oocyte ($1n$) and one small first polar body (which disintegrates).

At what stage of meiosis does a secondary oocyte arrest until fertilization?

Metaphase II of Meiosis II.

What stimulus causes a secondary oocyte to complete Meiosis II?

Penetration/fusion by a sperm cell.

What two products result when a secondary oocyte completes Meiosis II?

One haploid ovum ($1n$) and a second polar body.

Define nondisjunction.

An error in chromosome segregation where homologous chromosomes or sister chromatids fail to separate properly.

Distinguish euploidy from aneuploidy.

Euploidy is having a normal, expected complete set of chromosomes ($n$, $2n$); Aneuploidy is differing from expected by one or a few chromosomes.

What are the chromosome formulas for nullisomy, monosomy, and trisomy?

Nullisomy = $2n - 2$; Monosomy = $2n - 1$; Trisomy = $2n + 1$.

What gamete proportions are produced if nondisjunction of a chromosome pair occurs in Meiosis I?

All gametes are abnormal: $50\%$ are $n + 1$ (trisomic-producing) and $50\%$ are $n - 1$ (monosomic-producing).

What gamete proportions are produced if nondisjunction of sister chromatids occurs in Meiosis II?

$50\%$ normal ($n$), $25\%$ trisomic-producing ($n + 1$), and $25\%$ monosomic-producing ($n - 1$).

Which autosomal trisomies are viable at birth in humans?

Trisomy 13 (Patau syndrome), Trisomy 18 (Edward syndrome), and Trisomy 21 (Down syndrome).

Why are no human newborns observed with autosomal monosomies?

Because autosomal monosomies create lethal gene dosage imbalances early during embryonic development.

In a cell where $2n = 16$, how many total chromosomes will be present under monosomy, trisomy, and nullisomy?

Monosomy = $15$ chromosomes ($2n - 1$); Trisomy = $17$ chromosomes ($2n + 1$); Nullisomy = $14$ chromosomes ($2n - 2$).

What percentage of normal human brain neurons can exhibit aneuploidy?

Up to $30\%$ of neurons.

What chromosomal abnormality causes Down syndrome?

Trisomy 21 ($2n + 1 = 47$).

Define polyploidy.

A chromosomal condition in which an organism possesses complete extra sets of chromosomes ($3n$, $4n$, $5n$, etc.).

Differentiate autopolyploidy from allopolyploidy.

Autopolyploidy involves additional chromosome sets derived from the same species; Allopolyploidy involves additional chromosome sets derived from different species.

Why are polyploids with odd chromosome set numbers (e.g. $3n$, $5n$) usually sterile?

Because homologous sets cannot pair equally during Meiosis I, producing unbalanced, inviable gametes.

How are commercial seedless triploid ($3n = 33$) bananas and watermelons produced?

By crossing a tetraploid ($4n$) parent plant (producing $2n$ gametes) with a diploid ($2n$) parent plant (producing $1n$ gametes).

What is the ploidy level and genomic composition of modern bread wheat?

It is an allohexaploid ($6n$) consisting of chromosome sets from three species, represented as AABBDD.

What chemical disrupts spindle fiber formation to induce polyploidy artificially?

Colchicine.

Identify the four main categories of structural chromosome rearrangements.

Duplication (segment doubled), Deletion (segment lost), Inversion (segment reversed), and Translocation (segment moved to nonhomologous chromosome).

What is haploinsufficiency?

A phenomenon where a single wild-type allele in a deletion heterozygote does not produce enough gene product to give a normal phenotype.

What chromosomal deletion mutation causes Williams-Beuren syndrome?

Deletion of a region containing $17$ genes on chromosome 7 due to unequal crossing over at PMS repeat markers.

Distinguish paracentric inversions from pericentric inversions.

Paracentric inversions do NOT include the centromere, while pericentric inversions DO include the centromere.

What abnormal chromosome structures are generated when crossing over occurs in a paracentric inversion loop during meiosis?

One dicentric chromosome (two centromeres) that forms a bridge and breaks, and one acentric fragment (no centromere) that is lost.

What is the difference between reciprocal and nonreciprocal translocations?

In reciprocal translocations, segments are exchanged between two nonhomologous chromosomes; in nonreciprocal translocations, a segment moves to a nonhomologous chromosome without an exchange.